The invention relates to a chair, more especially an office chair, comprising a seat plate which is arranged on an undercarriage and a backrest which in the manner of a so called synchronous mechanism permit a synchronous movement of backrest and seat plate.
Such chairs are known in numerous constructional forms. They have a coordinated movement of backrest and seat plate. This means that when the user leans against the backrest the latter and the seat plate assume a position which corresponds substantially to a lying or leant back position. The result of such a synchronous movability is that when working the user can sit upright in a forwardly inclined position whereas in the leant back position he assumes a position of rest.
Such a movement coupling of seat plate and backrest is generally referred to as synchronous mechanism. Chairs which are equipped with such a mechanism have the advantage that they adjust themselves substantially automatically to the weight of the user without any manual adjustment being necessary. The cooperation of seat and backrest in the sense described takes place independently of the weight of the user.
The hitherto known chairs with synchronous mechanism already afford a relatively high sitting comfort. However, the hitherto known chair constructions cannot adjust themselves to load shifts from one side of the seat surface to the other or to forces acting on one side of the backrest. However, such unequal loads frequently occur in the use of office chairs. For ergonomic reasons it would be desirable to accommodate corresponding loads so that the chair can adapt itself to a laterally changed posture of the user. Known chairs however do not afford any such possibilities.
The problem underlying the present invention is therefore to provide a chair having a synchronous mechanism of the type mentioned at the beginning which can adapt itself to different loads on both sides of the chair and in this manner further increases the sitting comfort.
This problem is solved according to the invention by a chair having the features of claim 1.
The seat plate of the chair according to the invention is mounted laterally tiltable, i.e. it can be tilted about a longitudinal axis of the chair, said axis extending substantially forwardly from the backrest. This tilting can be combined with a tilting movement about a transverse axis of the chair in the sense of the hitherto known synchronous mechanism. The seat plate is coupled to two backrest supports whose lower ends are pivotally mounted on the undercarriage in such a manner that that they can be pivoted independently of each other and can assume different inclinations. By the movement coupling between the seat plate and the supports said seat plate is pressed down on the same side by a rearward pivoting of the support whilst the opposite side thereof is raised.
This movement coupling represents a considerable extension of the hitherto known principle of the synchronous mechanism and corresponds to a greater extent to an ergonomic sitting position because crosswise acting loads on the seat plate and the backrest can be compensated by the mechanism of the chair.
According to a preferred embodiment of the invention the lower ends of the backrest supports are connected pivotally about a common horizontal axis to a base of the undercarriage and the tilt axis for the lateral tilting of the seat plate lies in a vertical plane between the backrest supports. Said base can be a part at the upper end of the central column of the undercarriage which is mounted vertically adjustably on the column and is stationary with respect to the movability of the backrest and the seat plate.
According to a further preferred embodiment on both sides of the seat plate bearings are provided which receive the lower ends of the backrest supports in a region behind the pivot axis thereof. Said bearings can for instance engage round the lower ends of the supports laterally and from below, for example such that the support ends lie in the bearings on resilient pads. If one of the two backrest supports is pivoted rearwardly about its horizontal axis a pressure is exerted downwardly on the bearing assembly and presses the seat plate downwardly on the corresponding side. This leads to the desired lateral tilting of the seat plate.
Preferably the seat plate is pivotal together with the lower ends of the backrest supports about the horizontal pivot axis thereof. Thus, in this case the seat plate can be not only laterally tilted but can also execute a pivot movement about a transverse axis of the chair so that on uniform load on the two backrest supports it can tilt down rearwardly.
Further preferably, in the rear region of the undercarriage a bearing member is mounted which is tiltable about the pivot axis of the backrest supports and supports the pivot shaft of the seat plate. This bearing member if thus tilted together with the backrest supports and the seat plate about a transverse axis.
According to a further preferred embodiment of the present invention the seat plate is arranged in the rear region of the seat surface and in the front region of the seat surface a front seat plate is arranged, and from said seat plate on both sides of the chair leg springs extend rearwardly and support the rear tiltable seat plate from below on both sides. The rear seat plate can then execute the tilting movement only against the resistance of the leg springs which when no load is present keep the seat plate in a balanced position.
According to a further preferred embodiment of the present invention the front seat plate is carried by a front bearing member whose rear end is articulately connected to a front end of the bearing member and whose front end is articulately connected to the front end of the base of the undercarriage lying therebelow via a connection member which at its rear end receives the pivot shaft of the supports. The front bearing member, the bearing member arranged therebehind and connected articulately thereto for supporting the rear seat plate and the front connecting member are then connected to the base to form a quadrilateral whose corners are formed by articulations which permit the front bearing member, the rear bearing member and the connecting member to be adjusted with respect to each other so that they can form different angles to each other.
According to a further preferred embodiment of the invention the lower ends of the backrest supports are formed by legs angled forwardly in L shape, the front portions of which are connected pivotally about substantially vertically directed axes of rotation to the remainder of the respective support and the front ends of which accommodate the horizontal pivot shafts of the supports. Thus, in this case the backrest supports are not only pivotal with respect to the undercarriage about a common horizontal axis; in addition, the spacing of their vertical legs can be adjusted by a pivoting thereof with respect to the front portions of the horizontal legs mounted on the undercarriage.
More preferably the backrest supports are connected in an upper region by a transverse member whose ends are articulately connected to the supports in such a manner that the spacing between the connection points on the supports is kept constant without otherwise restricting a relative movement of the supports with respect to each other. Said transverse member effects a movement coupling of the upper portions of the backrest supports to each other. If one of the supports is tilted this leads automatically to a tilt movement coupled thereto of the other support. Thus, a pressure load on one side acting on the backrest causes a counter pressure on the other side of the backrest.
According to a further preferred embodiment said transverse member is formed as a rod with variable length for adjusting the spacing between the connection points at which the rod is connected by ball joints to the supports. The length of the transverse member can thus be set by the user for adaptation to the user's own body dimensions.
Hereinafter a preferred example of embodiment of the invention will be described in detail with the aid of the drawings.